Why should chemical fertilizers not be applied frequently to vegetable plots?
Many vegetable growers believe that the more fertilizer applied to the soil, the better, but in fact frequent application of chemical fertilizers to vegetable plots has many harmful effects. Frequently applying chemical fertilizers to vegetable plots not only affects the quality of vegetables but also damages the soil. Therefore, the amount of chemical fertilizer should be minimized as much as possible, and it should be applied in combination with organic fertilizers.
Vegetables grown with long-term chemical fertilizer application, especially the "nitrogen-fertilizer vegetables" produced by excessive nitrogen fertilizer, have their edible parts such as stems and leaves seriously contaminated by nitrate, which causes the nitrate content in vegetables to increase several times over. Nitrate is easily reduced to nitrite in the human body, and nitrite is a highly toxic substance that can cause oxygen deficiency in human cells. If children eat such vegetables over a long period, it can cause intellectual decline and can induce cancer, posing an extremely great hazard. Therefore, in some developed countries it is now explicitly stipulated that nitrogen-fertilizer vegetables are prohibited from the market, so as to effectively guarantee the interests of consumers.
Fresh tender vegetables produced entirely with chemical fertilizers look tender in stem and plump in leaf, and those who do not know the facts assume they are of superior quality. In fact, such vegetables have excessive nitrate content, and they also tend to become moldy and deteriorate during piling and storage, increasing the content of toxic substances. According to measurements, during winter and spring storage, Chinese cabbage (napa cabbage) treated with excessive nitrogen fertilizer has a rot and damage rate more than 20% higher than Chinese cabbage treated with farmyard manure, and its nitrate content also increases several times over.
Long-term application of ammonium sulfate, potassium sulfate and nitrogen-containing chemical fertilizers to vegetable soil will destroy the soil structure, make the soil harden, lose its flexibility and elasticity, and reduce water permeability and air permeability. With long-term chemical fertilizer application, calcium ions on the soil colloids are continuously exchanged out, and under the leaching of rainwater and irrigation water, ammonium sulfate and calcium chloride are leached down to the lower soil layers, causing a calcium deficiency in the plow layer, destroying the soil structure, making the soil hardened, reducing fertility, resulting in poor vegetable growth and lower yields. Because a good soil structure, in addition to containing a large amount of organic matter, is generally also rich in calcium; a calcium-deficient soil, no matter how good its structure, is unstable and easily destroyed. Only soil containing a certain amount of calcium, with its calcium-based water-stable aggregate structure, is the most ideal soil, capable of producing high-yield, high-quality vegetables.
Frequent application of chemical fertilizers can cause acidification damage to the soil. After acidic chemical fertilizers such as ammonium sulfate, ammonium chloride, potassium chloride and superphosphate are applied heavily to vegetable plots year after year, they leave a large amount of acidic substances in the soil, turning neutral soil acidic and making acidic soil even more acidified, which both destroys the soil microbial flora and is unfavorable to the normal growth of most vegetables. Long-term chemical fertilizer application, especially repeated annual application of ammonium sulfate, potassium sulfate, potassium chloride and the like, can only increase the content of sulfate ions and chloride ions in the soil, and these two ions are both harmful to the soil; they not only harm vegetable growth but also cause secondary salinization of the soil, destroy the soil structure, and affect the absorption of other nutrient elements by vegetables. Long-term excessive application of chemical fertilizers to vegetable plots not only harms the vegetables but also hinders their absorption of other nutrient elements, causing deficiency symptoms. For example, excessive application of nitrogen-containing chemical fertilizers can cause calcium deficiency in vegetables; too much nitrate nitrogen can cause molybdenum deficiency; too much potassium reduces the availability of calcium, magnesium and boron; too much phosphorus reduces the availability of calcium, zinc and boron, and so on.
Therefore, vegetable plots should, to the greatest extent possible, use organic fertilizers or other non-chemical nitrogen fertilizers to replace chemical nitrogen fertilizers. On the premise of not affecting the normal growth and development of vegetables, organic fertilizers such as barnyard manure and chicken manure, as well as various cake fertilizers, should be used as much as possible to replace chemical nitrogen fertilizers, so as to prevent vegetable contamination and reduce the residual pollution and destructive effects of chemical fertilizers on the soil.